This patch prepares the tx path to send data on multiple tx queues. It
introduces per queue register adresses and uses them in the alx_tx_queue
structs.

There are new helper functions for the queue mapping in the tx path.

Based on the downstream driver at github.com/qca/alx
Signed-off-by: Tobias Regnery <tobias.regnery@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>